Move code to fold now() from Simplifier() to ConstEvaluator

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This is a follow on suggestion from @rdettai on #1153 https://github.com/apache/arrow-datafusion/pull/1153/files#r735437280

Namely the substitution of now() for the current value is better described as "constant evaluation" rather than "algebraic simplification" so it should be done in the ConstEvaluator code.

Describe the solution you'd like
Move code to fold now() from Simplifier() to ConstEvaluator